Cactus Music
What does the track meter show? 
Generally Sonars meters will show approximately what your interfaces meters are showing. 
It's possible they have purposely padded down the USB output so as not to go over during live recordings where the live sound might be on the edge of clipping. 
 
If the mixers peak light is active , but Sonars track meter is way down then that's proof. No big deal, it's digital so you can apply normalizing to bring it up to par. 
 
What you need to keep in mind is this is a Live Mixer first and an Audio interface second. It's optimized for live not recording. The recording feature is an add on to help sell the board to bands wishing a rough live recording of gigs. It is why I might be purchasing one. I'm not going to even dream it will replace a proper interface,, even though that would sort of be nice. 
 
Thanks for the info, and I agree it IS, in effect, padded down. I haven't found an option to adjust it,yet- so I used your suggestion and normalized the track. The MTK12 is perfect for my needs,plenty of in/out's and routing options, but the noisy headphone amp makes me cringe every session. The onboard effects I have zero love for, but it's so easy to use plugins within DAW and monitor them through the mixer, so no real complaint. I'm also getting attached to Sonar, I really like this DAW. Customer support has been excellent.
 
I had an opportunity to chat with Soundcraft about my complaints and they were very cool,open about taking criticism. They were aware and working on it, and were planning to address said issues in future MTK-series,in addition to adding channel inserts among some other things I can't recall.
 
Again, Thanks

Actually, I am starting to use/open SONAR 1st when I start songwriting demos more and more...may be a new car smell thing,but wow is an understatement.
 

What I have is a few songwriting Templates so I can open them and be recording within a minute. 
They are actually CWP files, not template files, because I have midi drum tracks I use in a few styles. I like playing to a simple drum pattern instead of the metronome. 
They have a vocal and a DI guitar track already armed to record. 
Learn about track templates to speed things up. 
You can quickly populate a blank project using track templates. 

So i ended up sending back the Sig 12 MTK. I was having trouble with input/output levels, the effects were decent but not anything really special and I found the routing cumbersome for my simple set up particularly with the ASIO drivers, which were pretty necessary latency-wise.
I was hoping for a solution that would primarily be an interface and a live mixer when needed maybe once a month. But as said this is a mixer first and an interface second and although a nice piece of work, that needs to be remembered.
I ended up exchanging for: TASCAM UH-7000  which had some great reviews.
Will have to work out another solution for live recording - maybe just a small analog mixer bounced down to stereo.

Pick up a Tascam multi channel inerface,
https://www.sweetwater.com/store/detail/US16x08?product_id=US16x08&campaigntype=shopping&campaign=aaShopping%2520-%2520Core%2520-%2520Studio%2520%26%2520Recording&adgroup=Studio%2520%26%2520Recording%2520-%2520Audio%2520Interfaces&placement=google&adpos=1o1&creative=213185928233&device=c&matchtype=&network=g&gclid=CjwKCAjwmK3OBRBKEiwAOL6t1DO88kMnKFFwdNnX9eslmueqoz0wuN84iuDnVmAq6YVcWkxsI5ygqRoCcF4QAvD_BwE
 
They go on sale for $199 sometimes, They are the same as my us1641 and then they made the us1800. My us1641 is perfect for live recordings. I have the 8 xlr and then I run various line level feeds into the other channels fed from the FOH mixer and stealing signals from DI boxees etc. I even record the keyboard as MIDI. So I make a 16 track plus midi. My Board ( Yamaha 01v) has SPDIF so I can use that, the new models give you analog for 15/16. 
Look for used us1641 or us1800 on E bay. They still write drivers for them and mine works equally as well as my Scarlett. 

I've been pretty happy with my ufx1204 usb so far, 'cepting occasional driver issues. Nothing is perfect tho.